A solid laser device is well known in the art in which a rod-shaped laser medium and a pumping lamp are positioned at the focal points of an elliptic reflecting box, respectively. In the solid laser device, a temperature distribution is established radially in the rod, so that the laser beam is subjected to the thermal lens effect and deteriorates in quality. This difficulty is serious for a high power laser. In order to overcome the difficulty, a solid laser device has been proposed in which a flat-plate-shaped laser medium (hereinafter referred to as "a slab-shaped laser medium", when applicable) is employed, and the optical path in the laser medium is zigzagged, so that thermo-optical distortions are canceled out in the laser medium.
One example of the above-described solid laser device has been disclosed by Japanese Patent Application (OPI) No. 254686/1985 (the term "OPI" as used herein means an "unexamined published application"). FIG. 1 is a sectional view showing the optical arrangement of the solid laser device. In FIG. 1, reference numeral 1 designates a slab-shaped laser medium having a pair of optically smooth surfaces 1a which are in parallel with each other, and end faces 1b forming an angle of inclination with the surfaces; 2, a total reflection mirror arranged near one of the end faces 1b of the laser medium 1; 3, a partially reflecting mirror arranged near the other end face 1b, forming a stable resonator with the total reflection mirror 2; 4, a lamp for applying a pumping light beam to the laser medium 1; 5, a reflecting box made of reflecting mirrors, the reflecting box 5 containing the lamp 4 and the laser medium 1; 6, the water in the reflecting box 5; 7, the optical axis of a laser beam; and 8, a laser beam outputted through the partially reflecting mirror 3. Further in FIG. 1, vectors P and S are defined with respect to the plane which is defined by the optical axis 7 and the incident surfaces 1b to the laser medium 1. The vector S is perpendicular to the plane, and the vector P is in parallel with the latter.
The solid laser device thus designed operated as follows. The output light beam of the lamp 4 is reflected in the reflecting box 5, and is absorbed by the laser medium 1 to excite the latter. As a result, the laser medium 1 emits a light beam. The light beam is reflected by the total reflection mirror 2. The light beam thus reflected enters the laser medium after being refracted at the end face 1b of the laser medium 1. In the laser medium 1, the light beam is repeatedly reflected by the upper and lower surfaces 1a, thus reaching the other end face 1b. At the end face 1b, the light beam is refracted again, and advances to the partially reflecting mirror 3. The light beam reflected from the partially reflecting mirror 3 returns to the optical axis 7. Therefore, the light beam is amplified while reciprocating along the optical axis 7. When the light beam is amplified to a predetermined degree, then part of the light beam passes through the partially reflecting mirror 3; that is, it is taken, as a laser beam, out of the stable resonator.
In the above-described solid laser device with the slab-shaped laser medium 1, the optically smooth surfaces of the laser medium 1 are kept cooled at all times. In the laser medium, the light beam advances zigzag being reflected by the upper and lower optically smooth surfaces; that is, laser beam passes alternately through the surface region of the laser medium which is at low temperature and the middle region of the laser medium which is at high temperature. Accordingly, the laser beam is not subjected to thermal lens effect, and a high power laser output can be stably obtained.
In the solid laser device described above, the laser medium 1 is generally rectangular; that is, the width in the direction S is about two to five times the thickness in the direction P. Therefore, the laser beam has a high order mode of rectangle shape, reflecting the section of the laser medium 1. For instance, in a solid laser device in which its laser medium 1 is a YAG (Yttrium Aluminum Garnet) crystal emitting a laser beam of 1.06 .mu.m, the mode number in the direction P is several tens, and that in the direction S is several hundreds, and the beam divergence angle is up to several tens of milli-radians (mrad); that is, the laser beam is not sufficiently concentrated. Thus, while the above-described solid laser device can output a high power laser beam stably, the output laser beam is not sufficiently concentrated. That is, it is impossible for the laser beam to form a fine light spot. Accordingly, the application of the solid laser device to precision laser machining is limited.
Accordingly, an object of this invention is to eliminate the above-described difficulties accompanying a conventional solid laser device. More specifically, an object of the invention is to provide a solid laser device which can output a high power laser beam with excellent convergence, and can be applied to fine laser machining operations.